As a measure to improve the durability of a bearing part, a carbonitriding process of introducing carbon and nitrogen into a surface layer portion of a bearing part prior to quenching is known (see for example Japanese Patent Laying-Open Nos. 5-118336 (PTD 1) and 2012-31456 (PTD 2)). This carbonitriding process is known to improve the rolling fatigue life of a rolling bearing, particularly the life of the rolling bearing in an environment in which hard foreign matters intrude into the rolling bearing (foreign-matters intrusion environment). The carbonitriding process is a process in which a bearing part made of a steel is heated to a temperature range of the A1 transformation temperature or more in an atmosphere containing ammonia, carbon monoxide, carbon dioxide, and hydrogen, to thereby introduce carbon in a surface layer portion of the bearing part, or introduce nitrogen in the surface layer portion while suppressing decarburization of the surface layer portion.
Regarding this carbonitriding process, it is proposed to adjust ac* defined by the following formula (1) and adjust α defined by the following formula (2) to thereby keep the carbon concentration in the surface layer portion at an appropriate level and improve the rate at which nitrogen is introduced, and accordingly enhance the efficiency of the carbonitriding process (see for example Japanese Patent Laying-Open No. 2007-277648 (PTD 3)).
                              a          c          *                =                                            (              Pco              )                        2                                K            ×                          Pco              2                                                          (        1        )            Pco: partial pressure (atm) of carbon monoxidePco2: partial pressure (atm) of carbon dioxideK: <C>+CO2⇔ equilibrium constant of 2CO
                    α        =                                            P              N                                      0.006              ×                                                (                                      P                    H                                    )                                                  3                  2                                                              ×                                    (                              1.877                -                                  1.055                  ×                                      a                    c                    *                                                              )                        100                                              (        2        )            